1. FIELD OF THE INVENTION
This invention relates to a speed detecting apparatus suitable for use for controlling the speed of a rotating body or a moving body in a digital manner.
2. DESCRIPTION OF THE PRIOR ART
For the detection of the rotational speed of, for example, an electric motor as a digital signal, an encoder is generally used which generates a pulse signal whose frequency is proportional to the rotational speed of the motor. The encoder generates one pulse each time it rotates by an amount corresponding to 1/m (m: a large integer) of one complete revolution. A train of such pulses generated from the encoder is processed to detect the rotational speed of the motor. According to a known method as, for example, disclosed in JP-A-57-144465 (corresponding to U.S. Pat. No. 4,584,528), the rotational speed of a motor is detected with high accuracy by counting the number of pulses generated from an encoder mechanically coupled to the motor, and measuring the time in synchronous relation with the counting operation counting the number of the pulses. However, this method has the problem that speed detection is impossible in a low speed range where the output pulse frequency of the encoder is very low. A method to deal with such a problem is reported in the Society for the Research of Industrial Power Applications of the Institute of Electrical Engineers of Japan, IA-85-11 (1985). According to the reported method, an encoder generating a sine-wave signal is used, and the rotational speed of an associated motor is detected by detecting the rotational phase of the encoder from the analog output signal of the encoder and computing the rotational speed on the basis of a variation of the rotational phase. Although the latter method is advantageous in that the rotational speed can be detected with high resolution even in a low speed range, difficulty is encountered in high-accuracy speed detection due to, for example, distortion of the waveform of the sine-wave signal. Thus, the former method is defective from the aspect of the resolution in a low speed range, and the latter method is defective in the lack of consideration of the accuracy of speed detection. It has therefore been difficult to detect rotational speed with high resolution and high accuracy over a wide speed range, according to the prior art methods.